I've gone for: And yet back under the open sky, the fall of the light and corrosive pumice was feared.

Then we have the following line, quam tamen faces multae variaque lumina solabantur, which is relatively straightforward except that "lumen" is masculine and yet has a neuter ending, which "varia" presumably agrees with. Can "lumen" be of two genders?

Moving along, Deinde flammae flammarumque praenuntius, odor sulpuris, alios in fugam vertunt, excitant illum, is again relatively simple except that plural verbs are in agreement with a singular noun. I presume though that they are agreeing with the sense of "fires and flame", rather than with "odor sulpuris" in a strict sense.

On lumen - all three sources I've checked (Wheelock, Whitaker's Words, L&S) give it as neuter, following the pattern for 3rd decl nouns ending in -men (c.f carmen and nomen), so there's no issue there, surely.

I've gone for: And yet back under the open sky, the fall of the light and corrosive pumice was feared.

One correction: exesorum is a perfect passive participle, so it literally means "eaten through" rather than "corrosive". He's referring to the porousness of pumice stones.

I think quamquam modifies the two adjectives it precedes, rather than the whole clause, i.e. "Back under the open sky, the falling of pumice stones, though light and porous, was [still] feared." It's usually only at the beginning of sentences that quamquam means "and yet".

Einhard wrote:Then we have the following line, quam tamen faces multae variaque lumina solabantur, which is relatively straightforward except that "lumen" is masculine and yet has a neuter ending, which "varia" presumably agrees with. Can "lumen" be of two genders?

Lumen is always neuter.

Einhard wrote:Moving along, Deinde flammae flammarumque praenuntius, odor sulpuris, alios in fugam vertunt, excitant illum, is again relatively simple except that plural verbs are in agreement with a singular noun. I presume though that they are agreeing with the sense of "fires and flame", rather than with "odor sulpuris" in a strict sense.

The two subjects are "flames" and "the precursor of flames". Odor sulpuris is in apposition to the latter.

cum maxime vera memorantur = when the facts are best remembered...that I set forth/took down everything I had been in the midst of, whatever I had heard, immediately, when the facts are best remembered.